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main reference

the various ways in which objects from a set may be selected, generally without replacement, to form subsets. This selection of subsets is called a permutation when the order of selection is a factor, a
combination when order is not a factor. By considering the ratio of the number of desired subsets to the number of all possible subsets for many games of chance in the 17th century, the French...

major reference

A set of
r objects selected from a set of
n objects without regard to order is called a
combination of
n things taken
r at a time. Because each
combination gives rise to
r! permutations, the number of
combinations, which is written (
n/
r), can be expressed in terms of factorials